This invention relates to an apparatus and method for measuring the concentration of particles in a gas. Such particles may comprise, for example, dust, soot, smoke, solid pollutants and the like. The apparatus and method of the invention are especially intended for protecting engines and other gas receiving devices from damage caused by the intake of air containing too much dust or other particulate matter. However, the instant apparatus and method have a variety of other applications; for example, they may be used for checking the efficiency of dust removal equipment used in certain environmental control systems (e.g. removal of fibers from the air in textile mills, or dust in grain elevators) or for monitoring environmental pollution by checking the levels of particulate materials in waste gases such as flue gases, automobile exhaust systems, cement kilns and power generation plants.
Internal combustion engines used to power tanks and other military and other vehicles require a large supply of clean air to ensure maximum engine performance and engine life and to reduce maintenance requirements. Air cleaning systems have been developed that will remove 99% of the particulate matter that is drawn into the air intake system. Such high efficiency air cleaning systems are multi-stage units which include barrier type air filters. However, a simple dust leak in the air cleaning system (caused by, for example, accidental perforation of one of the air filters) can negate the effectiveness of the system. The vehicle operator must know when such a leak occurs so that he can shut off the engine and take steps to correct the problem before irreparable damage is done. This problem is especially of concern on military tanks equipped with gas turbine engines that are highly susceptible to damage by dust in the air under some operating conditions. Also, problems with excessively dusty air may be encountered in other internal combustion engines equipped with turbochargers. A reliable dust detection system can also be valuable on any engine and on many other systems where filters are used to remove dust from the air, either to provide a supply of clean air for a particular device or area or to control dust emission from a manufacturing operation or power generation system.
One prior art apparatus for measuring the concentration of particles in a gas is described in our U.S. Pat. No. 4,312,180 issued Jan. 26, 1982. This prior art apparatus passes particulate-carrying gas past first and second electrically conductive members spaced apart in a first region so as to provide a potential gradient in part of the region between the first and second members of at least about 30 kilovolts per centimeter. The first and second members thus form a corona and produce ions that charge a substantial proportion of the particles in the gas. Third and fourth electrically conductive members are spaced apart in a second region downstream from the first region and an electrical potential is applied between the third and fourth members so as to provide a potential gradient of substantially less than 30 kilovolts per centimeter therebetween. The charged particles produced in the first region are attracted to either the third or fourth electrically conductive member and there release their charge so that the current flowing from the third and fourth conductive members measures the concentration of particulate matter in the gas. Although this apparatus performs quite well, it does require the provision of two separate volatage sources. In addition, the apparatus cannot easily be retrofitted to existing pipework or similar ducts.
Another prior art apparatus for measuring the concentration of particles in a gas is described in U.S. Pat. No. 3,679,973 issued July 25, 1972 to N. S. Smith et al. In this prior art apparatus, the particulate-carrying gas is passed through a narrow throat, within which is located an ionizer which serves to charge the particles in the gas in substantially the same way as the first and second electrically conductive members in our own prior art apparatus described above. The air carrying the particles thus charged emerges from the throat into a wider duct, thereby reducing the rate of flow of the gas along the duct. The walls of wide duct are formed of a plurality of cylindrical, electrically-conductive members insulated from one another and each connected to a separate current measuring device. The reduction in flow velocity along the duct permits the particles to move radially of the duct onto the electrically-conductive members forming the walls of the duct, where they give up their charge to the electrically-conductive members, thereby causing a current flow through each measuring device. By measuring the current flow through the various current measuring devices, the concentration and size of the particles in the gas flow can be measured. This prior art apparatus is complicated; the specific apparatus shown in the drawings of the patent requires four separate electrically-conductive members with associated insulators therebetween, four current measuring devices and the voltage source necessary to charge the particles in the throat.
This invention seeks to provide an apparatus and method for measuring the concentration of particles in a gas which overcomes the aforementioned disadvantages of prior art apparatus and methods for this purpose. The invention also seeks to provide a method for retrofitting an apparatus for measuring the concentration of particles in a gas into an existing duct or similar structure.